In communications networks, there may be a challenge to obtain good performance and capacity for a given communications protocol, its parameters and the physical environment in which the communications network is deployed.
For example, for future generations of mobile communications systems frequency bands at many different carrier frequencies could be needed. For example, low such frequency bands could be needed to achieve sufficient network coverage for terminal devices and higher frequency bands (e.g. at millimeter wavelengths (mmW), i.e. near and above 30 GHz) could be needed to reach required network capacity. In general terms, at high frequencies the propagation properties of the radio channel are more challenging and beamforming both at the network node at the network side and at the terminal devices at the user side might be required to reach a sufficient link budget.
The terminal devices and/or the transmission and reception point (TRP) of the network node could implement beamforming by means of analog beamforming, digital beamforming, or hybrid beamforming. Each implementation has its advantages and disadvantages. A digital beamforming implementation is the most flexible implementation of the three but also the costliest due to the large number of required radio chains and baseband chains. An analog beamforming implementation is the least flexible but cheaper to manufacture due to a reduced number of radio chains and baseband chains compared to the digital beamforming implementation. A hybrid beamforming implementation is a compromise between the analog and the digital beamforming implementations. As the skilled person understands, depending on cost and performance requirements of different terminal devices, different implementations will be needed.
For terminal devices the incoming signals might arrive from all different directions. Hence it is beneficial to have an antenna implementation at the terminal devices which has the possibility to generate omni-directional-like coverage in addition to the high gain narrow beams.
However, for antenna arrays with single polarized antenna elements (and implementing analog beamforming) it could be challenging to generate beams with a large variety in beamwidths. Hence, antenna arrays with dual-polarized elements are preferred such that dual-polarized beamforming as disclosed in WO2011/050866 A1 and WO2016141961 A1 can be used to generate beams with basically any beamwidths, ranging from the beamwidth of a single antenna element to the beamwidth of the entire antenna array.
One issue with using dual-polarized beamforming, where both polarization ports are used to create a port, for an antenna array implementing analog beamforming is that the resulting beam will only have one distinct polarization in each direction, which means that polarization mismatch can occur between the transmitter and receiver, which can deteriorate the link significantly.
FIG. 1 schematically illustrates an antenna arrangement 100 having two baseband chains 110a, 110b. Each baseband chain 110a, 110b is operatively connected to antenna elements 130a, 130b of an antenna array 120, where one of the baseband chains is operatively connected to all antenna elements of a first polarization, and the other of the baseband chains is operatively connected to all antenna elements of a second polarization.
Even though the antenna arrangement 100 has one baseband chain per polarization, only one signal for one polarization can be transmitted/received in each direction at a given time. It is envisioned that the number of baseband chains might possibly be reduced whilst at the same time applying dual-polarized beamforming for an antenna array implementing analog beamforming, where a single baseband chain is connected to all antenna elements of both polarizations of the antenna array. In this way dual-polarized beamforming can be applied whilst only using a single baseband chain. Such an antenna arrangement might be cheaper to manufacture compared to the antenna arrangement illustrated in FIG. 1. But if two orthogonal polarizations are desired simultaneously with the envisioned antenna arrangement, two such antenna arrangements have to be stacked next to each other, which adds implementation space.